A sound sleep is dreamless. Dreams require a certain expenditure of nerve force and mental energy, so that dreamless sleep is the most restful. Disagreeable dreams and nightmares are generally associated with indigestion and biliousness, which also occasion a general restlessness.

Maybe it’s true that digestion problems can trigger dreams, but I know from experience that nightmares easily arise without such triggers. Now in my seventh decade, my dreams are worse, and the same is true of my wife’s. I’m convinced we both have a degree of PTSD, having had plenty of stressful episodes in our adventurous lives. So far as I know, there has been no definitive scientific study of the phenomenon, but one thing I can vouch for: strong emotions will set memories in permanence and then provide the basis for nightmares.